1. Field of the Invention
The present invention relates to an alarm circuit, particularly though not exclusively for use with a smoke or fire barrier
2. Description of the Related Art
In this specification we use the following terms and meanings:    “curtain” means a textile screen that extends down from a deployment mechanism;    “barrier” means the apparatus including both the curtain and the deployment mechanism.
Our smoke or fire barriers generally comprise:                a gravity fail safe curtain having an electric lift motor and an electromagnetic brake at the motor, the brake being for normally holding the curtain upon application of voltage to it and for fail safe release of it in absence of voltage;        one or a plurality of alarm circuits, each having a normally closed, fire or smoke switch,        a control circuit to which the or all of the alarm circuits is/are connected, the control circuit being adapted:                    to apply voltage to the brake for holding curtain up and            remove the voltage for release of the curtain, when the or one of the alarm circuits is open circuit.                        
We refer to such a barrier as a “barrier of the type defined”.
A problem can arise in that one of the alarm circuits can remain closed even in the event of its fire or smoke detecting switch functioning correctly and opening. The problem is caused by some extraneous event such as a fixing penetrating the cable of the circuit and short-circuiting its pair of wires. This would not normally be detected, prior to an emergency, because of the normal state of the switch being closed.